1. Field of the Invention
The present invention relates to an image forming utilizing electrophotography, such as a printer, a copier machine and a facsimile machine.
2. Description of the Related Art
In a conventional image forming apparatus utilizing electrophotography, developing means adheres toner to an electrostatic latent image which is formed on a photosensitive member by exposure means, a toner image is accordingly formed and then transferred onto a transfer paper, and the toner image formed on the transfer paper is fixed by fixing means on the transfer paper. Known in particular as an apparatus which permits to form a color image is an image forming apparatus in which a toner image which is formed on a photosensitive member is primarily transferred onto an intermediate transfer medium and thus primarily transferred image which is on the intermediate transfer medium is secondarily transferred onto a transfer paper.
There are two typical types of color image forming apparatuses. An image forming apparatus of one of the two types comprises one photosensitive member for example. In such an image forming apparatus, toner images of different colors are created one after another on the photosensitive member while primarily transferring each toner image onto an intermediate transfer medium every time a toner image in each color is formed, a color toner image, which is an overlap of the toner images of the plurality of colors, is accordingly formed on the intermediate transfer medium, and thus formed color toner image is secondarily transferred onto a transfer paper, whereby a color image is obtained.
Known as an image forming apparatus of the other type is an image forming apparatus of the so-called tandem type in which a plurality of photosensitive members are disposed in a direction of rotational driving of an intermediate transfer medium which rotates such that the photosensitive members are faced with the intermediate transfer medium. In such an image forming apparatus, toner images in different colors are created on the respective photosensitive members, thus formed toner images are primarily transferred on the rotating intermediate transfer medium so as to superimpose the toner images on top of the other, and a color toner image resulting from the superimposition is secondarily transferred onto a transfer paper.
By the way, primary transfer described above is realized as a primary transfer bias is applied between an intermediate transfer medium and a photosensitive member for instance, while secondary transfer described above is realized as a secondary transfer bias is applied between the intermediate transfer medium and a secondary transfer member, which is disposed to face with the intermediate transfer medium, with a transfer paper for instance interposed between the intermediate transfer medium and the secondary transfer member.
Among known as this type requires to output different primary transfer bias values for the different colors, or change an output value of the primary transfer bias or the secondary transfer bias in accordance with an environmental condition such as a temperature, a humidity level, etc.
In a configuration that the primary transfer bias is controlled to a constant voltage, since a potential difference is maintained constant in a primary transfer unit, even when an output value of the secondary transfer bias changes during application of the primary transfer bias, the change does not influence primary transfer almost at all.
On the contrary, when an output value of the primary transfer bias changes during application of the secondary transfer bias, since an electric field between the intermediate transfer medium and the secondary transfer member changes, there is a risk that secondary transfer will become instable. Particularly in a configuration that the intermediate transfer medium comprises a plurality of layers including a conductive layer, since application of the primary transfer bias upon the intermediate transfer medium is application upon the entire intermediate transfer medium which is not limited to the primary transfer unit but also includes the secondary transfer unit, the change of the output value of the primary transfer bias exerts a large influence over secondary transfer.
Hence, it is preferable to change the output value of the primary transfer bias in accordance with various conditions, since the materials of toner in the respective colors are different, since the toner in the respective colors is accumulated on the intermediate transfer medium, since the transfer efficiency changes because of a change in temperature or humidity, or for other reasons. It is also preferable to determine the timing of changing the output value of the primary transfer bias so that the timing will not adversely affect secondary transfer. However, the output value of the primary transfer bias needs be determined before starting primary transfer.
An image forming apparatus of the tandem type, a monochrome image forming apparatus or the like which comprises one photosensitive member in particular often uses a structure that an intermediate transfer medium moves passed a primary transfer unit immediately after moving passed a secondary transfer unit, in an attempt to reduce the size of the apparatus. Therefore, next primary transfer starts while secondary transfer is still ongoing depending on the size of a transfer paper, and the output value of the primary transfer bias will change during execution of secondary transfer but for countermeasure. Noting this, it is important to set up the timing of changing the output value of the primary transfer bias in such a manner that secondary transfer will not be negatively influenced.